Phengaris alcon, the Alcon blue or Alcon large blue, is a butterfly of the family Lycaenidae and is found in Europe and across the Palearctic to Siberia and Mongolia.
Description from Seitz
L. alcon Schiff. (= areas Esp., euphemus Godt.) (83 a). Large, the male above deep blue, but without brilliant gloss. The female black-brown, dusted with dark blue in the basal area. The dark violet-grey underside has numerous ocelli. L. alcon is easily distinguished from the following species (coeligena [L. coeligena Oberth. China], euphemus, arcas, arion, arionides ...) by the male bearing on the blue disc of the forewing no other black spots but the discocellular lunule. Central Europe and North Asia, from the coast of the North Sea (Hamburg, Bremen, Belgium) to the Mediterranean, and from France to the Altai, Dauria and Tibet, ab. nigra Wheel, has the males strongly darkened, the females being quite black above. In ab. cecinae Hormuz. the ocelli of the underside are absent or strongly reduced. In ab. pallidior Schultz the margin is grey instead of black. — marginepunctata Gillm. has a row of black dots before the margin, almost parallel with it; found by Hafner at Loitsch and other places in Carniola.— In the form rebeli Hirschke the blue of the upperside is more brilliant and more extended, the dark margin being reduced, in the female only the apical area black; Styria. — monticola Stgr. (83 a) has a narrow black margin like rebeli, but the blue is very deep and dark, so dull as in true alcon; from the Alps of Switzerland and the Caucasus. — Egg white, finely reticulated, laid on the flowers of the food-plant (Gentiana pneumonanthe). The larva generally does not break through the shell on the upperside, so that the holes of empty eggs are not easily noticed. At first grey, later on reddish brown with dark dorsal line and dark head. The butterflies occur on damp meadows where Gentiana grows; they are plentiful in such places, sometimes even in abundance, from the end of May into July, in the North not before the end of June.
Taxonomy
There are five subspecies: P. a. alcon (Central Europe) P. a. jeniseiensis (Shjeljuzhko, 1928) (southern Siberia) P. a. sevastos Rebel & Zerny, 1931 (Carpathians) P. a. xerophila Berger, 1946 (Central Europe) †P. a. arenaria (Netherlands)
There has been controversy over whether Phengaris rebeli, currently regarded as an ecotype within the Alcons, should be listed as a separate species. The two types are morphologically indistinguishable and molecular analysis has revealed little genetic difference, mostly attributable to localized habitat adaptation. Still some maintain that they should be treated as distinct species, especially for conservation purposes, because they parasitise different host ant colonies and parasitise these ants at different rates, and also rely on different host plant species (Gentiana pneumonanthe in the case of Phengaris alcon and Gentiana cruciata in the case of Phengaris rebeli).
Ecology
The species can be seen flying in mid- to late summer. It lays its eggs onto the marsh gentian (Gentiana pneumonanthe); in the region of the Alps they are sometimes also found on the related willow gentian (Gentiana asclepiadea). The caterpillars eat no other plants.
Parasitic relationship
Like some other species of Lycaenidae, the larval (caterpillar) stage of P. alcon depends on support by certain ants; it is therefore known as a myrmecophile.